Wheatstone bridge-type transducers with reduced thermal shift

Abstract

Thermal shift in Wheatstone bridge-type transducers is reduced by providing a diaphragm of semiconductor material accepting formation of strain gages therein having a uniform temperature coefficient of resistance therethrough. That diaphragm is supported in a mechanically floating condition, such as by interposing a cushion of compliant material between the diaphragm and a support which is subject to mechanical strains. An amount of unbalance required for balancing the strain gages on the diaphragm is determined. One of these strain gages is provided with a series balance equal to one-half of that determined amount, while an adjacent one of the strain gages is provided with a shunt balance equivalent to one-half of that determined amount. Alternatively, the strain gages are provided with an initial thermal zero shift offset requiring only a shunt balance for compensation, and thermal zero shift is then compensated only with that shunt balance, or the strain gages are provided with an initial thermal zero shift offset requiring only a series balance of compensation, and that thermal zero shift is then compensated with only that series balance.

I claim: 
     
       1. In a method of making a Wheatstone bridge-type transducer having a housing, the improvement of reducing thermal shift, comprising in combination the steps of: providing a diaphragm of semiconductor material accepting formation of strain gages therein having a uniform temperature coefficient of resistance therethrough;   providing in said diaphragm strain gages, while maintaining said uniform temperature coefficient of resistance;   providing for said diaphragm a support subject to mechanical strains;   providing a cushion of compliant material capable of isolating said diaphragm from said mechanical strains to which said support is subject;   interposing said cushion of compliant material between said diaphragm and said support, when supporting said diaphragm in said housing with the aid of said support, while leaving said strain gages exposed to strains imposed by a physical parameter to be measured with said transducer;   providing electric lead wires for said strain gages with sufficient limpness to prevent transmission of mechanical strains therealong;   determining an amount of unbalance required of said strain gages for balancing a zero offset of said strain gages;   providing one of said strain gages with a series balance equal to one-half of said determined amount; and   completing balancing of said zero offset and also avoiding a thermal zero shift of the Wheatstone bridge by providing an adjacent one of said strain gages with a shunt balance equivalent to one-half of said determined amount.   
     
     
       2. A method as claimed in claim 1, including the steps of: providing a wafer of said semiconductor material;   determining a region in said wafer having a uniform temperature coefficient of resistance therethrough; and   dissecting said region from said wafer to provide said diaphragm having said uniform temperature coefficient therethrough.   
     
     
       3. A method as claimed in claim 1, wherein: said cushion is made of an elastomer having a compliance sufficient to isolate said diaphragm from said mechanical strains.   
     
     
       4. A method as claimed in claim 1, wherein: said cushion is made of a silicone rubber having a compliance sufficient to isolate said diaphragm from said mechanical strains.   
     
     
       5. A method as claimed in claim 1, including the step of: covering said diaphragm and strain gages with electrically insulating gel.   
     
     
       6. A method as claimed in claim 5, wherein: a silicone gel is selected as said gel covering said diaphragm and strain gages.   
     
     
       7. A method as claimed in claim 5, including the step of: providing said cushion with a cavity conforming in shape to an overall configuration of said diaphragm.   
     
     
       8. In a method of making a Wheatstone bridge-type transducer having a housing, the improvement of reducing thermal shift, comprising in combination the steps of: providing a diaphragm of semiconductor material accepting formation of strain gages therein having a uniform temperature coefficient of resistance therethrough;   providing in said diaphragm strain gages with an initial zero offset and thermal zero shift requiring only a shunt balance for compensation; and   compensating zero offset and thermal zero shift of said strain gages with only said shunt balance.   
     
     
       9. A method as claimed in claim 8, including the step of: making both said initial zero offset and thermal zero shift positive.   
     
     
       10. A method as claimed in claim 8, including the step of: making both said initial zero offset and thermal zero shift negative.   
     
     
       11. In a method of making a Wheatstone bridge-type transducer having a housing, the improvement of reducing thermal shift, comprising in combination the steps of: providing a diaphragm of semiconductor material accepting formation of strain gages therein having a uniform temperature coefficient of resistance therethrough;   providing in said diaphragm strain gages with an initial zero offset and thermal zero shift requiring only a series balance for compensation; and   compensating thermal zero shift of said strain gages with only said series balance.   
     
     
       12. A method as claimed in claim 11, including the steps of: making one of said initial zero offset and thermal zero shift positive; and   making the other of said initial zero offset and thermal zero shift negative.   
     
     
       13. In a Wheatstone bridge type transducer having a housing, the improvement of reducing thermal shift, comprising in combination: a diaphragm of semiconductor material having strain gages therein and a uniform temperature coefficient of resistance therethrough, said strain gages requiring a predetermined amount of unbalance for balancing a zero offset of said strain gages, and one of said strain gages having a series balance equal to one-half of said predetermined amount;   a support for said diaphragm, subject to mechanical strains;   means for isolating said diaphragm from said mechanical strains to which said support is subject, including a cushion of compliant material between said diaphragm and said support for supporting said diaphragm in said housing, while leaving said strain gages exposed to strains imposed by a physical parameter to be measured with said transducer;   electric lead wires for said strain gages having sufficient limpness to prevent transmission of mechanical strains therealong; and   means for completing balancing of said zero offset while also avoiding a thermal zero shift of the Wheatstone bridge, including a shunt balance equivalent to one-half of said predetermined amount, in an adjacent one of said strain gages.   
     
     
       14. A transducer as claimed in claim 13, including:  electrically insulating gel covering said diaphragm and strain gages in said housing.   
     
     
       15. A transducer as claimed in claim 13, wherein: said cushion comprises an elastomer having a compliance sufficient to isolate said diaphragm from said mechanical strains.   
     
     
       16. A transducer as claimed in claim 13, wherein: said cushion comprises a silicone rubber having a compliance sufficient to isolate said diaphragm from said mechanical strains.   
     
     
       17. A transducer as claimed in claim 14, wherein: said gel is a silicon gel covering said diaphragm and strain gages.   
     
     
       18. A transducer as claimed in claim 14, wherein: said cushion has a cavity conforming in shape to an overall configuration of said diaphragm.   
     
     
       19. In a method of making a Wheatstone bridge-type transducer having a housing, the improvement of reducing thermal shift, comprising in combination the steps of: providing a diaphragm of semiconductor material accepting formation of strain gages therein having a uniform temperature coefficient of resistance therethrough;   providing in said diaphragm strain gages and determining an amount of unbalance required for balancing a zero offset of said strain gages;   supporting said diaphragm in a mechanically floating condition in said housing;   providing one of said strain gages with a series balance equal to one-half of said determined amount; and   completing balancing of said zero offset and avoiding also a thermal zero shift of said bridge by providing an adjacent one of said strain gages with a shunt balance equivalent to one-half of said determined amount.   
     
     
       20. In a Wheatstone bridge-type transducer, the improvement of reducing thermal shift, comprising in combination: a diaphragm of semiconductor material having strain gages therein and a uniform temperature coefficent of resistance therethrough, said strain gages having an initial zero offset and thermal zero shift requiring only a shunt balance for compensation; and   means for compensating zero offset and thermal zero shift of said strain gages including only said shunt balance.   
     
     
       21. In a Wheatstone bridge-type transducer, the improvement of reducing thermal shift, comprising in combination: a diaphragm of semiconductor material having strain gages therein and a uniform temperature coefficient of resistance therethrough, said strain gages having an initial zero offset and thermal zero shift requiring only a series balance for compensation; and   means for compensating zero offset and thermal zero shift of said strain gages including only said series balance.   
     
     
       22. In a Wheatstone bridge-type transducer having a housing, the improvement of reducing thermal shift, comprising in combination: a diaphragm of semiconductor material having strain gages therein and a uniform temperature coefficient of resistance therethrough, said strain gages requiring a predetermined amount of unbalance for balancing a zero offset of said strain gages;   means for supporting said diaphragm in a mechanically floating condition in said housing;   a series balance equal to one-half of said predetermined amount for one of said strain gages; and   means for completing balancing of said zero offset while also avoiding a thermal zero shift of the Wheatstone bridge, including a shunt balance equivalent to one-half of said predetermined amount connected to an adjacent one of said strain gages.